There is proposed a technique of adjusting a sound pressure level of reproduced sound in an acoustic space such as a vehicle compartment. For example, Patent Reference 1 proposes a technique of correcting a level of reproduced sound at each frequency band by an equalizer thereby to adjust the sound pressure level in accordance with the acoustic space and a listening position. Also, Patent Reference 2 proposes a method of arranging a speaker array including a plurality of speakers and controlling a phase and a sound volume of a sound signal outputted by each speaker, thereby to create a high sound pressure area at a certain position.
Further, Patent References 3 and 4 disclose methods of adjusting the level of sound reproduced by speakers arranged in front of and behind a listening position in an acoustic space.